1. Field of the Invention
This invention relates to digital data recovery, and, particularly, to recovery of digital data from storage devices employing serialized data encoding techniques.
There is a need for reliably recovering digital data from the storage medium of a mass storage device, such as a disk drive where the data is stored in a serialized format. Encoded data is generally represented by a pulse received at a controller indicating a state transition (e.g. a flux reversal) registered on the storage medium. Various encoding schemes are known for data storage and retrieval wherein both data and timing information are encoded in a serialized bit stream. Examples are phase modulation (PM), frequency modulation (FM), modified frequency modulation (MFM), and double modified frequency modulation (M.sup.2 FM). All of these techniques require the synchronization of a master clock with internal clock signals embedded in the data stream. The normal detection technique involves the provision of a signal detection window for examining a signal in the form of a pulse or a state transition. Depending upon the encoding scheme, the signal detected during the signal detection window can indicate a particular data value or clock synchronization signal.
Heretofore, typical data recovery techniques have involved the provision of a data detection window centered on a synchronized timing cell during which window a data signal can be expected. Conversely, a period called a clock detection window is provided between data detection windows. During the clock detection window, the timing clock signal is expected.
The clock detection window is, typically, the complement of the data detection window. It bridges bit cell timing units and is centered on the transition between adjacent timing units.
There are many factors contributing to the degradation of signals which can impair the reliable recovery of encoded data. One type of degradation is called peak shifting. Another type of degradation is called pulse peak suppression (or reduction). The degradation is usually caused by overlapping pulses or signals from the magnetic mediums which are consequences of physical characteristics of the storage system, such as the magnetic storage media and/or the magnetic circuitry of read and write heads of disk drives. Peak shifting in particular can be predicted and partially corrected by write precompensation techniques. There is nevertheless a need to improve the reliability of data recovery even in the absence of write precompensation.
A preferred encoding technique is Modified Frequency Modulation (MFM). The characteristics of the MFM encoding technique are well-known. The rule is essentially to provide a transition (or pulse) at the center of the bit cell timing unit for a one (or data bit) and a transition at the leading boundary of a bit cell for a zero (or clock bit) except that a clock bit is not used immediately after the occurrence of a data bit. The object of the coding is to provide a minimum spacing of at least one bit cell timing unit between transitions (pulses) and a maximum spacing of no greater than two bit cell timing units between transitions. A further practical limitation for accurate data recovery is that the peak shift of each bit can be no greater than one-quarter of one bit cell timing unit.
2. Description of the Prior Art
Prior art data recovery techniques used in most known data storage systems, and the like, utilize a framing window with a fixed width or duration. The framing window is defined as a particular voltage level. At one level the data pulse is detected while at another level the clock pulse is detected. Typically, this requires centering data pulses and clock pulses in the appropriate window (data or clock) by the use of a phased locked loop which locks onto incoming data pulses and clock pulses in a feedback loop. Predictable shifts in data and clock pulses have been partially accounted for by write precompensation and read postcompensation whereby the pulses are advanced or delayed in order to constrain occurrences thereof to the standard framing windows. However, it is possible for the data and/or clock pulses to be shifted beyond the limits of the proper positions of the fixed-width framing window. What is needed is a method whereby the windows can be adapted to the predicted position of the input data and clock pulses (or bits).